1. Field of the Invention
This invention relates to reflecting mirrors used in optical systems in general and, in particular, to a form of actuator which may be used to correct the pointing direction of a mirror or other optical component and/or to induce distortions in the reflecting surface of a mirror, for example, in a deformable mirror.
2. Summary of the Prior Art
The use of electrodistortive actuators, for example actuators made of stacks of lead magnesium niobate interleaved with electrically conducting planes, is known in the prior art. Devices of the foregoing construction are useful for positioning segments of a segmented mirror to insure that the optical reflecting surface of the mirror does not have any discontinuities and that the reflecting surfaces of all elements comprising the mirror are uniformly directed toward a common location. Additional uses for such devices are in deformable mirrors where it is desired to introduce known distortions into an optical reflecting surface to either correct aberrations resulting from an incoming signal passing through a distorting medium, for example the Earth's atmosphere, or to introduce known distortions into an optical signal, for example a laser beam, prior to transmitting it through an optically distorting medium. Examples of applications in which electrodistortive actuators are used for distorting or changing the position of reflecting surfaces may be had by reference to U.S. Pat. No. 4,674,848 and 4,655,563 owned by the owner of the present invention and the teachings of which are incorporated herein by reference.
Electrodistortive actuators, for example actuators manufactured from stacks of lead magnesium niobate (PMN), are well-suited for use in effecting small changes to a large number of areas of a deformable mirror's reflecting surface in that they are easily manufactured with relatively small cross section, consequently permitting high density packing of the actuators. They suffer, however, from the inability to provide the large strokes required for inducing significant displacement of heavy optical components, e.g. the segments of a large, segmented astronomical telescope mirror. Simply making the stack longer to increase stroke becomes prohibitive in that long stacks tend to bend when activated against high resistive forces, i.e. stiff mirrors, and so the expected linear displacement is lessened. Thus, while PMN actuators can be controlled with a high degree of precision to produce very small movements, which is desirable when accurate control of a mirror's reflecting surface is required, they are not suitable for use in applications where large stroke is required.